Polyimide is a high heat-resistant resin which is typically prepared by reacting dianhydride with diamine in an organic solvent and subjecting the resulting polyamic acid, a precursor of polyimide, to thermal or chemical imidization.
With excellent in thermal resistance, chemical resistance, electrical insulation, and mechanical properties, polyimide resins find numerous applications in the electric and electronic appliance, adhesive, composite material, fiber, and film industries.
By virtue of its linear backbone structure which allows chains to be packed at a high density and by virture the rigidity of the imide ring itself, polyimide can show superior thermal resistance. But, such structural features make it difficult for the polyimide to dissolve in solvents and to be melted by heating, so that the polyimide is poor in processability and adhesiveness to other materials.
Particularly, the polyimide which is specialized to be used in areas where high temperature stability is required, as in the production of films, has a linear backbone structure such that the packing density of polymer chains is high, largely determining the thermal resistance of the polyimide. Commercially available polyimide films, exemplified by Kapton and Upilex, typically exhibit such structures. Kapton is known to be prepared from pyromellitic dianhydride (PMDA) and oxydianiline (ODA) monomers while Upilex can be prepared from 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (BPDA) and para-phenylenediamine (PPD) monomers. Also, it is known that a polyimide resin which is of higher thermal resistance can be obtained from a combination of PMDA and PPD monomers. However, very high rigidity and chain packing density of these polyimide resins brings about a bad effect upon their processability, flowability at high temperatures and adhesive properties.
To improve such problems, many attempts have been made, including introduction of polar groups into polymer backbones or side chains, introduction of bulky linking groups or side chains into backbones, and improvement of polymer backbone flexibility.
An improvement in the solubility of polyimide resins can be found in Macromolecules, 1994, 27, 1117, by Kurosaki et al., in which alicyclic acid anhydride is used as a monomer to prepare a soluble polyimide coating solution. Cyclic diamine is also used to prepare a soluble polyimide as disclosed in Polymer Chem. Ed., 1993, 31, 2345-2351, by Qin Jin et al. However, most of the soluble polyimides modified in these manners suffer from a difficulty in practical use because they have significantly degraded thermal stability and mechanical properties.
In order to improve the solubility and adhesiveness properties of polyimide, there was suggested the introduction of siloxane structures of diamine compounds into polymer backbones as in U.S. Pat. Nos. 5,859,181, 5,942,592 and 5,094,919. No matter how improved it is, the solubility property resulting from the introduction of siloxane structures of diamine compounds falls within the scope of the conventional polyimide films. In addition, the presence of a great amount of the siloxane structures in the polymer deteriorates the thermal resistance and mechanical properties of the polymer. It is also difficult to introduce a great amount of the siloxane structures into the polymer.